The invention relates to an instrument for determining the amounts of metabolic products in the blood by means of a radiation source and a radiation detection system delivering an output signal depending on the intensity of radiation of the aforesaid source after it has been affected by the blood containing the metabolic products.
Generally, the determination of amounts of metabolic products such as polypeptises, urea, cholesterol, glucose, CO.sub.2 or ethyl alcohol in the blood is carried out by withdrawing blood from the body and examining it chemically. However, the time required for ascertaining the particular amounts is relatively long and ranges from a few minutes to an hour.
On the one hand, there is danger in such extended periods of time that the blood may alter resulting in spurious results. On the other hand, a continuous testing of the metabolic products, as is desirable for instance when examining the glucose content when suspecting diabetes, or when determining the CO.sub.2 content during artificial respiration in the course of an operation, is impossible. Again, it is impossible, in current techniques, to ascertain transiently occurring, unknown metabolic products.
It is furthermore known that the metabolic products in the blood absorb infrared (IR) radiation, so that they may be ascertained by means of absorption measurements. However, such infrared absorption tests suffer from the difficulty that the blood acting as a solvent for the metabolic products in itself represents an aqueous solution which is strongly absorbing in the infrared spectrum, as is well known. Therefore, when performing measurements by means of previously known IR spectometers on the basis of transmission, very minute film thicknesses are required to obtain measurement signals that are useful at all. This requires in turn that the dissolved substances to be tested must be present at very high concentrations so that a relative change in absorption can be detected at all. Therefore only concentrations exceeding 1 percent can be ascertained in fact when using previously known IR spectrometers.
As shown by the article "Infrared Absorption Spectroscopy of Aqueous Solutions with a CO.sub.2 Laser", Applied Physics, Magazine 7, pp. 287-293 (1975), the measurement sensitivity in infrared absorption tests using the transmission mode has been significantly improved by employing lasers with an essentially higher intensity than the previously known light sources. When lasers are used, however, there frequently occurs the undesired side effect of appreciable heating of the substance to be examined due to the strong absorption properties of aqueous solutions. This problem is rather easily met when testing aqueous solutions in inorganic materials available in ample amounts of solution. However, the tests are significantly more difficult if the same transmission measurements must be carried out for blood, which is available only in lesser amounts and which furthermore already denatures when heated to 45.degree. C.
As was shown by applicant in an article in the book Modern Techniques in Physiological Sciences, Academic Press, London and New York, 1973, blood tests may also be carried out in vivo by means of laser beams. In that experiment, venous blood was passed in an extracorporeal shunt through a cuvette at a film thickness of 0.1 mm and at a flow rate of 30 cc per minute, and examined by means of a CO.sub.2 laser beam of 2 watts. It was found that the temperature of the blood being tested could be kept below the critical temperature limit because of its high flow rate, and that +/-0.5% changes in concentrations in ethanol or glucose could readily be shown. However, this method suffers from the drawback that the examination is exceedingly costly and practically is suited only for large operation. This method furthermore suffers significantly from the problems of achieving even flow through very thin cuvettes and then cleansing of same.
Again the ATR (Attenuated Total Reflectance) method described by J. Fahrenfort in Molecular Spectroscopy, Proceedings of a Conference at Brighton, 1968, Elsevier Publishing Company, Amsterdam, pp. 111-130, has already been used. In this method, the radiation with which to examine a sample is so beamed into a suitable plate as to be totally reflected several times at oppositely located surfaces of this plate before being made to pass out of it and examined for changes in intensity. The sample to be tested touches one or both sides of the plate totally reflecting the beam.